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Spontaneous vesicle phase formation by pseudogemini surfactants in aqueous solutions† Nan Sun,a Lijuan Shi,b Fei Lu,a Shuting Xiea and Liqiang Zheng*a The phase behavior of a kind of pseudogemini surfactant in aqueous solutions, formed by the mixture of sodium dodecyl benzene sulfonate (SDBS) and butane-1,4-bis (methylimidazolium bromide) ([mim-C4mim]Br2) or butane-1,4-bis(methylpyrrolidinium bromide) ([mpy-C4-mpy]Br2) in a molar ratio of 2 : 1, is reported in the present work. When [mim-C4-mim]Br2 or [mpy-C4-mpy]Br2 is mixed with SDBS in aqueous solutions, one cationic [mim-C4-mim]Br2 or [mpy-C4-mpy]Br2 molecule “bridges” two SDBS molecules by noncovalent interactions (e.g. electrostatic, p–p stacking, and s–p interactions), behaving like a pseudogemini surfactant. Vesicles can be formed by this kind of pseudogemini surfactant, determined by freeze-fracture transmission electron microscopy (FF-TEM) or cryogenic-transmission
Received 14th March 2014 Accepted 17th April 2014
electron microscopy (cryo-TEM) and dynamic light scattering (DLS). The mixed system of sodium dodecyl sulfate (SDS) with [mim-C4-mim]Br2 or [mpy-C4-mpy]Br2 was also constructed, and only micelles were observed. We infer that a pseudogemini surfactant is formed under the synergic effect of electrostatic, p–p stacking, and s–p interactions in the SDBS/[mim-C4-mim]Br2/H2O system, while
DOI: 10.1039/c4sm00565a
electrostatic attraction and hydrophobic interactions may provide the directional force for vesicle
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formation in the SDBS/[mpy-C4-mpy]Br2/H2O system.
Introduction Surfactants can self-assemble into a multitude of organized molecular assemblies such as micelles (spherical, wormlike, disk-shaped), lamellar phase (vesicle, sponge, and planar lamellar), and liquid crystals.1–5 The main driving force for the self-assembly of amphiphiles arises from weak interactions, including van der Waals, hydrophobic, screened electrostatic, p–p stacking, hydrogen-bonding interactions, etc.6–8 The organized molecular assemblies present many potential applications such as catalysts for organic synthesis,9 drug delivery syetems,1012 microreactors,13 and selective membranes.14 In particular, vesicles have attracted great deal of attention due to their potential application for the encapsulation and a
Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China. E-mail: [email protected]; Fax: +86-53188564750; Tel: +86-531-88366062
b
Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, China
† Electronic supplementary information (ESI) available: Surface tension curves of SDBS plotted against the surfactant concentrations (C) at different salt concentrations (Fig. S1); plots of the CMC values of SDBS and gCMC values against the concentrations of added salt (Fig. S2); surface tension curves of [mim-C4-mim]Br2-(SDBS)2 and [mpy-C4-mpy]Br2-(SDBS)2 plotted against the surfactant concentrations (C) at 25 C (Fig. S3); surface properties and micellization parameters of [mim-C4-mim]Br2-(SDBS)2 and [mpy-C4-mpy] Br2-(SDBS)2 and the calculation of the packing parameter P of SDBS/[mim-C4-mim]Br2/H2O and SDBS/[mpy-C4-mpy]Br2/H2O system (Table S1); DLS results for [mim-C4-mim]Br2-(SDS)2 and [mpy-C4-mpy]Br2-(SDS)2 aggregates (Fig. S4 and Table S2). See DOI: 10.1039/c4sm00565a
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controlled release of substances, such as drugs in pharmaceutical applications, avors and nutrients in foods, fragrances and dyes in cosmetics and textiles, etc.15–19 The common routes to obtain spontaneous vesicle formation are the mixture of cationic–anionic surfactants, the mixture of anionic–nonionic surfactants and/or the addition of external additives.20–24 For instance, spontaneous vesicle formation was observed in sodium dodecyl benzene sulfonate and benzylamine hydrochloride (SDBS/BzCl/H2O) in aqueous solution,25 1,4-bis(Ndodecyl-N,N-dimethylammonium)-butane dibromide and sodium benzoate (12-4-12/C6H5COONa/H2O) in aqueous solution and 2,3-dihydroxyl-1,4-bis(N-dodecyl-N,N-dimethylammonium)-butane dihydroxyl and sodium benzoate (12-4(OH)2-12/ C6H5COONa/H2O) in aqueous solution.22 Hao et al.26 have found that the mixture of nonionic tetraethylene glycol monododecyl ether (C12EO4) and dialkyl anionic sodium bis(2-ethyl hexyl) sulfosuccinate (AOT) in aqueous solution can self-assemble into onion-like vesicles. Our group5 has demonstrated that the p–p stacking interactions brought about by the ionic liquid 1-butyl3-methylimidazolium 2-naphthalenesulfonate (bmimNsa) combined with the hydrophobic effect and electrostatic attractions induce a phase transition in 1-dodecyl-3-methylimidazolium bromide (C12mimBr) from micelles to vesicles. Applying noncovalent interactions is a possible convenient approach to construct desired organized molecular assemblies. Zhang and co-workers have made great advances in the construction of supramolecular amphiphiles employing noncovalent interactions including hydrogen bonding, metal– ligand coordination, host–guest recognition, charge transfer
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interaction, p–p interactions, electrostatic attraction and have reported comprehensive reviews27–29 in this area. Very recently, “pseudogemini surfactants”, constructed through noncovalent interactions between a gemini-type molecule and single-chain surfactant, have attracted increasing attention. Feng et al.30 have developed a novel pH-switchable pseudogemini wormlike micelle system composed of N-erucamidopropyl-N,N-dimethylamine (UC22AMPM) and maleic acid in a molar ratio of 2 : 1 by electrostatic attraction. Feng et al.31 also reported a CO2switchable wormlike micelle system based on a pseudogemini surfactant constructed from the protonated N,N,N0 ,N0 -tetramethyl-1,3-propanediamine (TMPDA) molecule and sodium dodecyl sulfate (SDS). Wang et al.32 constructed a gemini-type surfactant system using the “gemini-type” organic salt 1,2-bis(2benzylammoniumethoxy)ethane dichloride (BEO) and sodium dodecyl sulfate (SDS). They reported that the gemini-type complexes were constructed through intermolecular electrostatic binding between the “gemini-type” organic salt and SDS, assisted by hydrophobic interactions and p–p interactions. Therefore, the reported pseudogemini surfactants were formed by noncovalent interactions such as electrostatic attraction, p–p stacking interactions or s–p3336 interactions, and can selfassemble into aggregates. Compared with the conventional gemini surfactant, pseudogemini surfactants formed by noncovalent interactions can effectively avoid complicated synthetic procedures and easily introduce stimuli-responsive substances or functional groups. Inspired by the pseudogemini concept, here we report a mixed system based on the anionic surfactant sodium dodecyl benzene sulfonate (SDBS) and butane-1,4-bis(methylimidazolium bromide) ([mim-C4-mim]Br2, Scheme 1a) or butane-1,4-bis(methylpyrrolidinium bromide) ([mpy-C4-mpy] Br2, Scheme 1b) with a precise stoichiometric ratio of 2 : 1. As a comparison, the systems of sodium dodecyl sulfonate (SDS) and [mim-C4-mim]Br2 or [mpy-C4-mpy]Br2 were also investigated. We expect that our work will help to achieve controllability of the structures and properties of ordered molecular aggregates by introducing special weak interactions.
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Experimental section Materials and sample preparation 1-Methylimidazole (98%) and D2O (99.96%) were purchased from Sigma-Aldrich. SDS (sodium dodecyl sulfonate) (99%) was obtained from Alfa Aesar. N-Methylpyrrolidine (98%) and 1,4dibromo-butane (98%) were the products of J&K Chemical Company. SDBS (sodium dodecyl benzene sulfonate) (95%) was purchased from Aladdin Reagent Company and recrystallized in methanol prior to use. Triply distilled water was used throughout all of the experiments. The [mim-C4-mim]Br2 and [mpy-C4-mpy]Br2 were synthesized in our laboratory and the purity of the products was ascertained by the 1H NMR spectra in DMSO and D2O respectively and the molecular structures are shown in Scheme 1. Synthesis of [mim-C4-mim]Br2 1-Methylimidazole (0.22 mol) and 1,4-dibromo-butane (0.1 mol) were dissolved in acetone and the mixture was stirred at room temperature under a nitrogen atmosphere and kept constant for 4 days. The obtained white mixture was ltrated and recrystallized at least three times with ethyl acetate and then dried in vacuo for 72 h. The purity was conrmed by the 1H NMR spectrum (400 MHz. DMSO, d/ppm): 1.79 (s, 2H, –NCH2CH2), 3.87 (s, 3H, –NCH3), 4.24 (s, 2H, –NCH2–), 7.74–7.82 (d, 2H, –NCHCHN–), 9.26 (s, 1H, –NCHN–). Synthesis of [mpy-C4-mpy]Br2 N-Methylpyrrolidine (0.22 mol) and 1,4-dibromo-butane (0.1 mol) were dissolved in acetone and the mixture was stirred at room temperature under a nitrogen atmosphere and kept constant for 3 days. The obtained white mixture was ltrated and recrystallized at least three times with ethanol–ethyl acetate and then dried in vacuo for 72 h. The purity of the product was conrmed by the 1H NMR spectrum (400 MHz. D2O, d/ppm): 1.77–1.81 (m, 2H, –NCH2CH2–), 2.11 (s, 4H, –CH2CH2–), 2.95 (s, 3H, –NCH3), 3.31 (s, 2H, –NCH2–), 3.31–3.43 (m, 4H, –NCH2–).
Apparatus and procedures Phase behavior The phase behavior was studied by visual inspection with the help of polarizers. Samples were prepared by mixing SDBS and [mim-C4-mim]Br2 or [mpy-C4-mpy]Br2 at a series of concentrations at the molar ratio of 2 : 1. Samples were mixed and equilibrated in a thermostatic bath for at least 4 weeks at 25 0.1 C until they were unchanged over an extended period of time. The stability of the sample and the phase boundaries was conrmed based on visual observations, with the help of crossed polarizers. Surface tension measurements
Scheme 1 Chemical structures of [mim-C4-mim]Br2 (a) and [mpy-C4mpy]Br2 (b).
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The surface tension measurements were carried out on a Model JYW-200B tensiometer (Chengde Dahua Instrument Co., Ltd., accuracy 0.1 mN m1) using the ring method. The temperature was controlled at 25 0.1 C using a thermostatic bath.
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Each sample was equilibrated for 15 min at the test temperature to reach equilibrium before the measurement.
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Polarized optical microscopy (POM) observations A polarized optical microscope (Panasonic Super Dynamic II WV-CP460) equipped with cooled CCD (Evolution MP5.1RTV, Qimaging, Canada) was used to observe the textures of the vesicles. The temperature was controlled at 25 0.1 C. A drop of sample solution was put into the special trough, which was then covered by a glass slip to avoid solution ow and evaporation. Dynamic light scattering The droplet size distributions of the micelles and the vesicle phase were determined by dynamic light scattering (DLS, DynaPro NanoStar, Wyatt Instrument Co.) with an argon-ion laser operating at 658 nm. All measurements were made at the scattering angle of 90 . The temperature was controlled at 25 C using a thermostat (F31C, Julabo) with an accuracy of 0.1 C. Freeze fracture transmission electron microscopy (FF-TEM) observations A small amount of the sample solution was placed on a 0.1 mm thick copper disk covered with a second copper disk. Then the copper sandwich containing the sample was plunged into liquid propane cooled by liquid nitrogen. Fracturing and replication were carried out on a Balzers BAF-400D equipment at 150 C. Pt/C was deposited at an angle of 45 . The replicas were examined on a JEOL JEM-1400 transmission electron microscope operated at 120 kV. The images were recorded on a Gatanmultiscan CCD and processed with a digital micrograph. Cryogenic transmission electron microscopy (cryo-TEM) observations The microstructures of vesicle solutions with low viscosity were determined by cryogenic transmission electron microscopy (cryo-TEM) observations. The samples were prepared in a controlled environment vitrication system (CEVS) at 25 C under 95% relative humidity. A micropipet was used to load 5 mL solutions onto a TEM copper grid, which was blotted with two pieces of lter paper, resulting in the formation of thin lms suspended on the mesh holes. Aer waiting for about 10 s, the samples were quickly plunged into a reservoir of liquid ethane (cooled by liquid nitrogen) at 165 C. The vitried samples were then stored in liquid nitrogen before being transferred to a cryogenic sample holder (Gatan 626) and examined with a JEOL JEM-1400 transmission electron microscope (120 kV) at about 174 C. The phase contrast was enhanced by underfocus. The images were recorded on a Gatanmultiscan CCD and processed with a digital micrograph.
Soft Matter
1.0 Hz. Once the linear viscoelastic region was determined, frequency sweep measurements were performed at a constant stress. The frequency region was from 0.01 to 100 Hz. All measurements were carried out at 25.0 C by using a Circulator HAAKE DC10 cyclic water bath (Karlsruhe, Germany). All samples for rheological measurements were prepared at least 4 weeks beforehand.
Results and discussion Phase behavior of the SDBS/[mim-C4-mim]Br2/H2O and SDBS/ [mpy-C4-mpy]Br2/H2O systems The phase behavior was determined mainly by visual observation, with the help of crossed polarizers. Fig. 1 shows the optical photographs of samples containing different amounts of SDBS and [mim-C4-mim]Br2, observed without (top) and with (bottom) crossed polarizers at 25 C. It is clear that with the increasing concentration of [mim-C4-mim]Br2, a series of phase changes can be observed. As shown in Fig. 1a, when the concentration of [mim-C4-mim]Br2 is 10 mM or lower, one can observe a transparent micelle phase with no birefringence. With increasing the concentration of [mim-C4-mim]Br2, a two-phase region with no birefringent texture appears until the concentration of [mim-C4-mim]Br2 reaches about 50 mM (Fig. 1b) and a two-phase region with a birefringent texture at the top appears at nearly 200 mM (Fig. 1d). When the concentration of [mim-C4mim]Br2 is in the range of about 300–500 mM (Fig. 1e–i), a birefringent, turbid, and viscous vesicle phase is observed. For the samples above 350 mM, POM observation shows Maltese crosses, indicating the existence of lamellar structures. A typical POM image is presented in Fig. 3. Within the vesicle region, samples with the concentration of [mim-C4-mim]Br2 above 375 mM become transparent and have birefringent textures that are different from the vesicles described above, indicating that the microstructure is different from the above vesicles. The samples in this region have a relatively higher viscosity than the samples of the above vesicle phase. However, the phase behavior of the SDBS/[mpy-C4-mpy]Br2/ H2O system is different from that of the SDBS/[mim-C4-mim] Br2/H2O system. As shown in Fig. 2, all of the samples are single phase. When the concentration of [mpy-C4-mpy]Br2 is 10 mM or
Rheological measurements Rheological measurements were performed on a Haake RS6000 rheometer with a coaxial cylinder sensor system (Z41 Ti). Dynamic oscillation-shear measurements were employed, in which the stress was varied while the frequency was kept at
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Fig. 1 Optical photographs of aqueous solutions with increasing amounts of [mim-C4-mim]Br2 and SDBS observed without (top) and with (bottom) crossed polarizers at 25 C.
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FF-TEM images of vesicle phase solutions for the SDBS/[mimC4-mim]Br2/H2O system: 350 mM (a) and 500 mM (b).
Fig. 4 Fig. 2 Optical photographs of aqueous solutions with increasing amounts of [mpy-C4-mpy]Br2 and SDBS observed without (top) and with (bottom) crossed polarizers at 25 C.
Cryo-TEM and FF-TEM images of vesicle phase solutions for the SDBS/[mpy-C4-mpy]Br2/H2O system: 50 mM (a, cryo-TEM) and 75 mM (b, FF-TEM). Fig. 5
Polarized micrograph for a typical sample of the 350 mM SDBS/ [mim-C4-mim]Br2/H2O system.
Fig. 3
lower (Fig. 2a), the solutions are clear with no birefringence, suggesting micelle solutions. When the concentration of [mpy-C4-mpy]Br2 is increased to about 50 mM, a turbid solution with birefringence appears. When the concentration is between 50 and 100 mM, a birefringent, turbid, bluish or ivory viscous phase is observed, indicating the appearance of a vesicle phase. Aer the vesicle region, samples with a concentration of [mpy-C4-mpy]Br2 above 100 mM become transparent and have birefringent textures different from that of the above samples. Microstructure observations and size distribution Cryo-TEM and FF-TEM are two powerful tools that can be employed to determine the self-assembled structure of surfactants in their hydrated states.37,38 For samples with low viscosity, cryo-TEM is a powerful tool to study the aqueous samples in their near-native hydrate state, while for samples with high viscoelasticity, FF-TEM is a more convenient tool for microscopical observation.39 The cryo-TEM and FF-TEM micrographs for selected samples are shown in Fig. 4 (SDBS/[mim-C4-mim] Br2/H2O system) and Fig. 5 (SDBS/[mpy-C4-mpy]Br2/H2O
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system). When the concentration of [mim-C4-mim]Br2 is 350 mM, polydisperse vesicles with diameters from about 50 nm to near 350 nm can be observed clearly (Fig. 4a). Slightly deformed vesicles can also be seen in the image, revealing the exibility of the vesicles, which is oen reported in hydrocarbon surfactant systems.40,41 When the concentration of [mim-C4-mim]Br2 increases to 500 mM, vesicles still mainly exist, which are densely packed (Fig. 4b). Similar to the SDBS/[mim-C4-mim]Br2/H2O system, vesicles can also be formed in the system of SDBS/[mpy-C4-mpy]Br2/ H2O. The cryo-TEM and FF-TEM observations for two selected vesicle phase samples are shown in Fig. 5. Cryo-TEM (Fig. 5a) shows that at the concentration of [mpy-C4-mpy]Br2 with 50 mM, vesicles with diameters about 200 nm are formed. When the concentration of [mpy-C4-mpy]Br2 increases to 75 mM, vesicles with diameters from about 100 nm to near 450 nm still mainly exist (Fig. 5b, FF-TEM image). Slight vesicle deformation can also be observed, indicating the exibility of vesicles.40,41 Dynamic light scattering (DLS) is utilized to further observe the aggregate size and morphology of the aqueous pseudogemini surfactant solution above the CMC (critical micelle concentration). All of the samples were chosen from the [mim-C4-mim]Br2-(SDBS)2 and [mpy-C4-mpy]Br2-(SDBS)2 systems described above. The DLS results are shown in Fig. 6, and the obtained hydrodynamic radii (R) of the aggregates from the DLS data are summarized in Table 1. Variations in aggregate size are observed when the concentrations of [mim-C4-mim]Br2
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concentration of [mpy-C4-mpy]Br2 increased. All of the sizes of the large aggregates are in agreement with the TEM results. However, as shown in Fig. S4,† the DLS results showed that only micelles with diameters lower than 20 nm formed in the SDS/[mim-C4-mim]Br2/H2O system and the SDS/[mpy-C4-mpy] Br2/H2O system. It can be concluded that the SDBS/[mim-C4mim]Br2/H2O system and the SDBS/[mpy-C4-mpy]Br2/H2O system experience a spontaneous phase transition from micelles to vesicles with increasing concentrations of the mixed solutions.
Rheological properties of the La-phase
Fig. 6 The size distributions of the [mim-C4-mim]Br2-(SDBS)2 (a) and [mpy-C4-mpy]Br2-(SDBS)2 (b) aggregates at different [mim-C4-mim] Br2 or [mpy-C4-mpy]Br2 concentrations. The hydrodynamic radius is plotted on a logarithmic scale.
Table 1 The obtained hydrodynamic radii (R) of [mim-C4-mim]Br2(SDBS)2 and [mpy-C4-mpy]Br2-(SDBS)2 aggregates versus the [mim-C4-mim]Br2 or [mpy-C4-mpy]Br2 concentrations
[mim-C4-mim]Br2-(SDBS)2 C (mM) R (nm)
1 7.0
5 8.6
10 15.8
350 232.6
425 220.2
500 182.7
10 13.6
25 64.8
50 113.5
75 214.7
[mpy-C4-mpy]Br2-(SDBS)2 C (mM) R (nm)
1 7.2
5 7.9
or [mpy-C4-mpy]Br2 increase. As can be seen from Fig. 6a, when the concentration of [mim-C4-mim]Br2 is lower than 10 mM, with increasing concentration of the mixture, the hydrodynamic radii (R) of the micelles increase slightly from 7.0 to 15.8 nm. When the [mim-C4-mim]Br2 concentration exceeds 350 mM, the hydrodynamic radius of the aggregates is about 200 nm. As for the SDBS/[mpy-C4-mpy]Br2/H2O system, the DLS results (Fig. 6b) show that micelles with a radius of about 10 nm formed when the concentration of [mpy-C4-mpy]Br2 was lower than 10 mM. Larger aggregates also formed when the
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Rheology is a powerful method not only to show the macroproperties of a surfactant system but also to conrm the microstructures of complex uids.42 Rheological measurements were carried out to characterize the macroscopic properties of the La-phase. The shear viscosity curves for the samples of the SDBS/[mim-C4-mim]Br2/H2O system in the La-phase as a function of shear rate are shown in Fig. 7. With increasing concentration of the pseudogemini surfactant, the viscosity of the solution becomes larger owing to the dense packing of the aggregates. It can be clearly seen from the gure that the viscosity decreases sharply with the increase of shear rate, showing emblematical non-Newtonian behavior of shear-thinning, which is typical for the La-phase.37,4344 Fig. 8 shows the dynamic rheological data for four selected samples at C[mim-C4-mim]Br2 ¼ 350, 400, 450 and 500 mM. As can be seen from the gure, the four samples have similar rheological properties. At C[mim-C4-mim]Br2 ¼ 350 mM, the storage modulus (G0 ) and loss modulus (G00 ) are nearly independent of the oscillatory frequency over the whole investigated frequency, exhibiting the obvious viscoelasticity and dominant elastic properties of the vesicle phase. When the concentrations of [mim-C4-mim]Br2 are 400, 450 and 500 mM, the storage modulus (G0 ) and loss modulus (G00 ) are slightly dependent on the frequency. Moreover, for all the selected samples, the storage modulus is almost 1 order of magnitude higher than the loss modulus. With increasing concentration of [mim-C4-mim] Br2, both the storage modulus (G0 ) and the loss modulus (G00 ) increase slightly. The complex viscosity rh*r decreases linearly with increasing frequency with a slope of 1. The rheological
Fig. 7 Viscosity as a function of shear rate for the vesicle phase
samples of the SDBS/[mim-C4-mim]Br2/H2O system: 350, 400, 450 and 500 mM.
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Fig. 8 Dynamic rheological data for the vesicle phase samples of SDBS/[mim-C4-mim]Br2/H2O: 350 (a), 400 (b), 450 (c) and 500 (d) mM.
results are consistent with the typical characteristics of densely packed vesicle solutions.45–48 Fig. 9 exhibits the rheological properties of the [mpy-C4-mpy] Br2 system at a [mpy-C4-mpy]Br2 concentration of 75 mM. Similar to the [mim-C4-mim]Br2 system, the solution demonstrates characteristic non-Newtonian behavior. Over the whole frequency, the storage modulus (G0 ) and the loss modulus (G00 ) 0 remain almost unchanged, and G is almost 1 order of magnitude higher than G00 . The complex viscosity rh*r decreases linearly over the whole frequency range with a slope of 1. In the SDBS/[mpy-C4-mpy]Br2/H2O system, vesicles are formed at a relatively lower concentration of the pseudogemini surfactant. Therefore, both the storage modulus and the loss modulus in the [mpy-C4-mpy]Br2 system are much lower.
Dynamic rheological data for the vesicle phase samples of the SDBS/[mpy-C4-mpy]Br2/H2O system: 75 mM.
Fig. 9
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Proposed mechanism of vesicle formation Surface tension measurements were carried out to explore the probable interactions between SDBS and [mim-C4-mim]Br2 or [mpy-C4-mpy]Br2. Surface tension curves of SDBS plotted against the surfactant concentration at different salt concentrations at 25 C are shown in Fig. S1.† The plots of the CMC and gCMC values for SDBS against the concentrations of added salts are shown in Fig. S2.† It is clear that with the increase of ionic strength, the CMC values of SDBS decrease sharply. A smaller value of gCMC means a greater ability to reduce the surface tension of a surfactant solution. As shown in Fig. S2(b),† the obtained gCMC values decrease sharply at rst and then change slightly with the increase of ionic strength considering the experimental errors. Therefore, the addition of [mim-C4-mim] Br2 and [mpy-C4-mpy]Br2 can distinctly reduce both the CMC and gCMC values. This result is mainly caused by the enhanced attractions between the head group of SDBS and [mim-C4mim]2+ or [mpy-C4-mpy]2+, including electrostatic, p–p stacking, and s–p interactions.5 The enhanced interactions can effectively reduce the electrostatic repulsions among the head groups of SDBS and then facilitate micelle formation.22 Very recently, Wang et al.32 have proved that in a 1,2-bis(2-benzylammoniumethoxy)ethane dichloride and sodium dodecyl sulfate (BEO/SDS) mixed aqueous solution, a (SDS)2-BEO gemini-type structure with enhanced aggregation ability can be fabricated through electrostatic binding between SDS and BEO. In the present SDBS/[mim-C4-mim]Br2/H2O and SDBS/[mpy-C4mpy]Br2/H2O systems, we infer that one [mim-C4-mim]Br2 or [mpy-C4-mpy]Br2 molecule may also bridge two SDBS molecules to form a pseudogemini surfactant molecule. As the
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concentration of the mixed solutions increases, the hydrophobic interactions among the SDBS alkyl tails become stronger, causing the alkyl tails to pack more tightly. As a result, a micelle-to-vesicle transition occurs when the concentration of the connecting molecule increases under the cooperative effect of electrostatic, p–p stacking, s–p, and hydrophobic interactions in the [mim-C4-mim]Br2/SDBS/H2O system and electrostatic, s–p, and hydrophobic interactions in the [mpy-C4-mpy] Br2/SDBS/H2O system. To further prove the weak interactions which induce the formation of vesicles, we also investigated the systems of SDS/[mim-C4-mim]Br2/H2O and SDS/[mpy-C4-mpy] Br2/H2O. The results show that all of the samples are clear, transparent, and homogeneous with no birefringence, indicating that only micelle solutions exist. Compared with SDBS, the surfactant SDS lacks a benzene ring on the molecule. 1 H NMR was carried out to explore the intermolecular interactions in the aqueous solutions of the pseudogemini surfactants. The 1H NMR spectra of the SDBS/[mim-C4-mim] Br2/H2O system at different concentrations in D2O at 25 C are shown in Fig. 10. It is clear that the protons around the headgroup move upeld slightly with the increase in concentration. Generally, the chemical shi of protons on surfactants moves downeld during aggregation in the absence of any other specic interactions.49,50 The upeld shis of the protons in the present system are probably caused by the p–p stacking. The addition of [mim-C4-mim]Br2 generates intermolecular p–p stacking between the phenyl and imidazolium rings, and the circular current makes the protons around the headgroup resonate at a higher eld due to the shielding effect.51 As the concentration of pseudogemini surfactant increases, the protons on the phenyl and imidazolium rings continue to shi upeld obviously, indicating the enhancement of p–p stacking interactions between the two aromatic rings. Meanwhile, the [mim-C4-mim]2+ may strongly combine with the SDBS headgroup, with the aromatic ring penetrating into the hydrophobic
Soft Matter
The possible mechanism of the phase transition for the pseudogemini surfactants system.
Scheme 2
cores of the pseudogemini surfactant aggregates, and then cause a phase transition from micelles to vesicles. In addition, some of the peaks broaden and even disappear, which is probably because the motion of the molecules is highly restricted in the vesicle phase.52,53 The packing parameter P is an important factor to interpret the self-assembled structure for a particular amphiphile. P is determined as P ¼ v/al by Israelachvili et al.54 where v is the effective hydrophobic chain volume, a is the effective headgroup area of the surfactant molecules, and l is the surfactant alkyl chain length. Surfactants with P below 1/3 tend to form spherical micelles, while P values between 1/3 and 1/2 prefer to form cylindrical aggregates and P values between 1/2 and 1 prefer to form bilayers. Surfactants with even higher values of P (P > 1) favor the formation of reverse structures. The calculated P value (Table S1†) is 0.56 for the SDBS/[mim-C4-mim]Br2/H2O system and 0.53 for the SDBS/[mpy-C4-mpy]Br2/H2O system, which are consistent with the formation of bilayer structures. With increasing concentration, the [mim-C4-mim]2+ cations would bind to the headgroup of SDBS strongly under the cooperative effect of electrostatic, p–p stacking and s–p interactions, and [mpy-C4-mpy]2+ cations would bind to the headgroup of SDBS under the cooperative effect of electrostatic and s–p interactions. Meanwhile, the hydrophobic interaction among the SDBS alkyl tails becomes stronger and makes the alkyl tails pack more tightly with increasing concentration. As a result, a phase transition from the micelle phase to the vesicle phase is induced at a certain concentration. According to the experimental results and analysis above, the possible mechanism of the phase transition from micelle phase to vesicle phase for the SDBS/[mim-C4-mim]Br2/H2O system and the SDBS/[mpy-C4-mpy]Br2/H2O system was drawn and is shown in Scheme 2. One can see that the strong interactions between SDBS and the cationic connecting molecule may provide the directional force for vesicle formation by pseudogemini surfactants.
Conclusions
Fig. 10 Partial proton assignments and 1H NMR spectra of the SDBS/ [mim-C4-mim]Br2/H2O system at different concentrations in D2O at 25 C.
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In summary, we have designed a kind of pseudogemini surfactants utilizing the anionic surfactant SDBS and the smallmolecule cationic spacers [mim-C4-mim]Br2 and [mpy-C4-mpy] Br2 at a molar ratio of 2 : 1. The presence of [mim-C4-mim]Br2 and [mpy-C4-mpy]Br2 induces vesicle formation in the
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pseudogemini surfactant system. We speculate that the electrostatic, p–p stacking and s–p interactions between SDBS and [mim-C4-mim]Br2, and the electrostatic and s–p interactions between SDBS and [mpy-C4-mpy]Br2 combined with the hydrophobic interaction among the SDBS alkyl tails contribute to the formation of vesicles. We believe that our research is helpful to achieve the controllability of organized assemblies by tuning specic weak interactions.
Acknowledgements The authors are grateful to the National Natural Science Foundation of China (no. 91127017), the National Basic Research Program (2013CB834505), Specialized Research Fund for the Doctoral Program of Higher Education of China (no. 20120131130003) and the Shandong Provincial Natural Science Foundation, China (ZR2012BZ001).
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Soft Matter
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PAPER
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Spontaneous vesicle phase formation by pseudogemini surfactants in aqueous solutions† Nan Sun,a Lijuan Shi,b Fei Lu,a Shuting Xiea and Liqiang Zheng*a The phase behavior of a kind of pseudogemini surfactant in aqueous solutions, formed by the mixture of sodium dodecyl benzene sulfonate (SDBS) and butane-1,4-bis (methylimidazolium bromide) ([mim-C4mim]Br2) or butane-1,4-bis(methylpyrrolidinium bromide) ([mpy-C4-mpy]Br2) in a molar ratio of 2 : 1, is reported in the present work. When [mim-C4-mim]Br2 or [mpy-C4-mpy]Br2 is mixed with SDBS in aqueous solutions, one cationic [mim-C4-mim]Br2 or [mpy-C4-mpy]Br2 molecule “bridges” two SDBS molecules by noncovalent interactions (e.g. electrostatic, p–p stacking, and s–p interactions), behaving like a pseudogemini surfactant. Vesicles can be formed by this kind of pseudogemini surfactant, determined by freeze-fracture transmission electron microscopy (FF-TEM) or cryogenic-transmission
Received 14th March 2014 Accepted 17th April 2014
electron microscopy (cryo-TEM) and dynamic light scattering (DLS). The mixed system of sodium dodecyl sulfate (SDS) with [mim-C4-mim]Br2 or [mpy-C4-mpy]Br2 was also constructed, and only micelles were observed. We infer that a pseudogemini surfactant is formed under the synergic effect of electrostatic, p–p stacking, and s–p interactions in the SDBS/[mim-C4-mim]Br2/H2O system, while
DOI: 10.1039/c4sm00565a
electrostatic attraction and hydrophobic interactions may provide the directional force for vesicle
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formation in the SDBS/[mpy-C4-mpy]Br2/H2O system.
Introduction Surfactants can self-assemble into a multitude of organized molecular assemblies such as micelles (spherical, wormlike, disk-shaped), lamellar phase (vesicle, sponge, and planar lamellar), and liquid crystals.1–5 The main driving force for the self-assembly of amphiphiles arises from weak interactions, including van der Waals, hydrophobic, screened electrostatic, p–p stacking, hydrogen-bonding interactions, etc.6–8 The organized molecular assemblies present many potential applications such as catalysts for organic synthesis,9 drug delivery syetems,1012 microreactors,13 and selective membranes.14 In particular, vesicles have attracted great deal of attention due to their potential application for the encapsulation and a
Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China. E-mail: [email protected]; Fax: +86-53188564750; Tel: +86-531-88366062
b
Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, China
† Electronic supplementary information (ESI) available: Surface tension curves of SDBS plotted against the surfactant concentrations (C) at different salt concentrations (Fig. S1); plots of the CMC values of SDBS and gCMC values against the concentrations of added salt (Fig. S2); surface tension curves of [mim-C4-mim]Br2-(SDBS)2 and [mpy-C4-mpy]Br2-(SDBS)2 plotted against the surfactant concentrations (C) at 25 C (Fig. S3); surface properties and micellization parameters of [mim-C4-mim]Br2-(SDBS)2 and [mpy-C4-mpy] Br2-(SDBS)2 and the calculation of the packing parameter P of SDBS/[mim-C4-mim]Br2/H2O and SDBS/[mpy-C4-mpy]Br2/H2O system (Table S1); DLS results for [mim-C4-mim]Br2-(SDS)2 and [mpy-C4-mpy]Br2-(SDS)2 aggregates (Fig. S4 and Table S2). See DOI: 10.1039/c4sm00565a
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controlled release of substances, such as drugs in pharmaceutical applications, avors and nutrients in foods, fragrances and dyes in cosmetics and textiles, etc.15–19 The common routes to obtain spontaneous vesicle formation are the mixture of cationic–anionic surfactants, the mixture of anionic–nonionic surfactants and/or the addition of external additives.20–24 For instance, spontaneous vesicle formation was observed in sodium dodecyl benzene sulfonate and benzylamine hydrochloride (SDBS/BzCl/H2O) in aqueous solution,25 1,4-bis(Ndodecyl-N,N-dimethylammonium)-butane dibromide and sodium benzoate (12-4-12/C6H5COONa/H2O) in aqueous solution and 2,3-dihydroxyl-1,4-bis(N-dodecyl-N,N-dimethylammonium)-butane dihydroxyl and sodium benzoate (12-4(OH)2-12/ C6H5COONa/H2O) in aqueous solution.22 Hao et al.26 have found that the mixture of nonionic tetraethylene glycol monododecyl ether (C12EO4) and dialkyl anionic sodium bis(2-ethyl hexyl) sulfosuccinate (AOT) in aqueous solution can self-assemble into onion-like vesicles. Our group5 has demonstrated that the p–p stacking interactions brought about by the ionic liquid 1-butyl3-methylimidazolium 2-naphthalenesulfonate (bmimNsa) combined with the hydrophobic effect and electrostatic attractions induce a phase transition in 1-dodecyl-3-methylimidazolium bromide (C12mimBr) from micelles to vesicles. Applying noncovalent interactions is a possible convenient approach to construct desired organized molecular assemblies. Zhang and co-workers have made great advances in the construction of supramolecular amphiphiles employing noncovalent interactions including hydrogen bonding, metal– ligand coordination, host–guest recognition, charge transfer
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Soft Matter
interaction, p–p interactions, electrostatic attraction and have reported comprehensive reviews27–29 in this area. Very recently, “pseudogemini surfactants”, constructed through noncovalent interactions between a gemini-type molecule and single-chain surfactant, have attracted increasing attention. Feng et al.30 have developed a novel pH-switchable pseudogemini wormlike micelle system composed of N-erucamidopropyl-N,N-dimethylamine (UC22AMPM) and maleic acid in a molar ratio of 2 : 1 by electrostatic attraction. Feng et al.31 also reported a CO2switchable wormlike micelle system based on a pseudogemini surfactant constructed from the protonated N,N,N0 ,N0 -tetramethyl-1,3-propanediamine (TMPDA) molecule and sodium dodecyl sulfate (SDS). Wang et al.32 constructed a gemini-type surfactant system using the “gemini-type” organic salt 1,2-bis(2benzylammoniumethoxy)ethane dichloride (BEO) and sodium dodecyl sulfate (SDS). They reported that the gemini-type complexes were constructed through intermolecular electrostatic binding between the “gemini-type” organic salt and SDS, assisted by hydrophobic interactions and p–p interactions. Therefore, the reported pseudogemini surfactants were formed by noncovalent interactions such as electrostatic attraction, p–p stacking interactions or s–p3336 interactions, and can selfassemble into aggregates. Compared with the conventional gemini surfactant, pseudogemini surfactants formed by noncovalent interactions can effectively avoid complicated synthetic procedures and easily introduce stimuli-responsive substances or functional groups. Inspired by the pseudogemini concept, here we report a mixed system based on the anionic surfactant sodium dodecyl benzene sulfonate (SDBS) and butane-1,4-bis(methylimidazolium bromide) ([mim-C4-mim]Br2, Scheme 1a) or butane-1,4-bis(methylpyrrolidinium bromide) ([mpy-C4-mpy] Br2, Scheme 1b) with a precise stoichiometric ratio of 2 : 1. As a comparison, the systems of sodium dodecyl sulfonate (SDS) and [mim-C4-mim]Br2 or [mpy-C4-mpy]Br2 were also investigated. We expect that our work will help to achieve controllability of the structures and properties of ordered molecular aggregates by introducing special weak interactions.
Paper
Experimental section Materials and sample preparation 1-Methylimidazole (98%) and D2O (99.96%) were purchased from Sigma-Aldrich. SDS (sodium dodecyl sulfonate) (99%) was obtained from Alfa Aesar. N-Methylpyrrolidine (98%) and 1,4dibromo-butane (98%) were the products of J&K Chemical Company. SDBS (sodium dodecyl benzene sulfonate) (95%) was purchased from Aladdin Reagent Company and recrystallized in methanol prior to use. Triply distilled water was used throughout all of the experiments. The [mim-C4-mim]Br2 and [mpy-C4-mpy]Br2 were synthesized in our laboratory and the purity of the products was ascertained by the 1H NMR spectra in DMSO and D2O respectively and the molecular structures are shown in Scheme 1. Synthesis of [mim-C4-mim]Br2 1-Methylimidazole (0.22 mol) and 1,4-dibromo-butane (0.1 mol) were dissolved in acetone and the mixture was stirred at room temperature under a nitrogen atmosphere and kept constant for 4 days. The obtained white mixture was ltrated and recrystallized at least three times with ethyl acetate and then dried in vacuo for 72 h. The purity was conrmed by the 1H NMR spectrum (400 MHz. DMSO, d/ppm): 1.79 (s, 2H, –NCH2CH2), 3.87 (s, 3H, –NCH3), 4.24 (s, 2H, –NCH2–), 7.74–7.82 (d, 2H, –NCHCHN–), 9.26 (s, 1H, –NCHN–). Synthesis of [mpy-C4-mpy]Br2 N-Methylpyrrolidine (0.22 mol) and 1,4-dibromo-butane (0.1 mol) were dissolved in acetone and the mixture was stirred at room temperature under a nitrogen atmosphere and kept constant for 3 days. The obtained white mixture was ltrated and recrystallized at least three times with ethanol–ethyl acetate and then dried in vacuo for 72 h. The purity of the product was conrmed by the 1H NMR spectrum (400 MHz. D2O, d/ppm): 1.77–1.81 (m, 2H, –NCH2CH2–), 2.11 (s, 4H, –CH2CH2–), 2.95 (s, 3H, –NCH3), 3.31 (s, 2H, –NCH2–), 3.31–3.43 (m, 4H, –NCH2–).
Apparatus and procedures Phase behavior The phase behavior was studied by visual inspection with the help of polarizers. Samples were prepared by mixing SDBS and [mim-C4-mim]Br2 or [mpy-C4-mpy]Br2 at a series of concentrations at the molar ratio of 2 : 1. Samples were mixed and equilibrated in a thermostatic bath for at least 4 weeks at 25 0.1 C until they were unchanged over an extended period of time. The stability of the sample and the phase boundaries was conrmed based on visual observations, with the help of crossed polarizers. Surface tension measurements
Scheme 1 Chemical structures of [mim-C4-mim]Br2 (a) and [mpy-C4mpy]Br2 (b).
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The surface tension measurements were carried out on a Model JYW-200B tensiometer (Chengde Dahua Instrument Co., Ltd., accuracy 0.1 mN m1) using the ring method. The temperature was controlled at 25 0.1 C using a thermostatic bath.
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Each sample was equilibrated for 15 min at the test temperature to reach equilibrium before the measurement.
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Polarized optical microscopy (POM) observations A polarized optical microscope (Panasonic Super Dynamic II WV-CP460) equipped with cooled CCD (Evolution MP5.1RTV, Qimaging, Canada) was used to observe the textures of the vesicles. The temperature was controlled at 25 0.1 C. A drop of sample solution was put into the special trough, which was then covered by a glass slip to avoid solution ow and evaporation. Dynamic light scattering The droplet size distributions of the micelles and the vesicle phase were determined by dynamic light scattering (DLS, DynaPro NanoStar, Wyatt Instrument Co.) with an argon-ion laser operating at 658 nm. All measurements were made at the scattering angle of 90 . The temperature was controlled at 25 C using a thermostat (F31C, Julabo) with an accuracy of 0.1 C. Freeze fracture transmission electron microscopy (FF-TEM) observations A small amount of the sample solution was placed on a 0.1 mm thick copper disk covered with a second copper disk. Then the copper sandwich containing the sample was plunged into liquid propane cooled by liquid nitrogen. Fracturing and replication were carried out on a Balzers BAF-400D equipment at 150 C. Pt/C was deposited at an angle of 45 . The replicas were examined on a JEOL JEM-1400 transmission electron microscope operated at 120 kV. The images were recorded on a Gatanmultiscan CCD and processed with a digital micrograph. Cryogenic transmission electron microscopy (cryo-TEM) observations The microstructures of vesicle solutions with low viscosity were determined by cryogenic transmission electron microscopy (cryo-TEM) observations. The samples were prepared in a controlled environment vitrication system (CEVS) at 25 C under 95% relative humidity. A micropipet was used to load 5 mL solutions onto a TEM copper grid, which was blotted with two pieces of lter paper, resulting in the formation of thin lms suspended on the mesh holes. Aer waiting for about 10 s, the samples were quickly plunged into a reservoir of liquid ethane (cooled by liquid nitrogen) at 165 C. The vitried samples were then stored in liquid nitrogen before being transferred to a cryogenic sample holder (Gatan 626) and examined with a JEOL JEM-1400 transmission electron microscope (120 kV) at about 174 C. The phase contrast was enhanced by underfocus. The images were recorded on a Gatanmultiscan CCD and processed with a digital micrograph.
Soft Matter
1.0 Hz. Once the linear viscoelastic region was determined, frequency sweep measurements were performed at a constant stress. The frequency region was from 0.01 to 100 Hz. All measurements were carried out at 25.0 C by using a Circulator HAAKE DC10 cyclic water bath (Karlsruhe, Germany). All samples for rheological measurements were prepared at least 4 weeks beforehand.
Results and discussion Phase behavior of the SDBS/[mim-C4-mim]Br2/H2O and SDBS/ [mpy-C4-mpy]Br2/H2O systems The phase behavior was determined mainly by visual observation, with the help of crossed polarizers. Fig. 1 shows the optical photographs of samples containing different amounts of SDBS and [mim-C4-mim]Br2, observed without (top) and with (bottom) crossed polarizers at 25 C. It is clear that with the increasing concentration of [mim-C4-mim]Br2, a series of phase changes can be observed. As shown in Fig. 1a, when the concentration of [mim-C4-mim]Br2 is 10 mM or lower, one can observe a transparent micelle phase with no birefringence. With increasing the concentration of [mim-C4-mim]Br2, a two-phase region with no birefringent texture appears until the concentration of [mim-C4-mim]Br2 reaches about 50 mM (Fig. 1b) and a two-phase region with a birefringent texture at the top appears at nearly 200 mM (Fig. 1d). When the concentration of [mim-C4mim]Br2 is in the range of about 300–500 mM (Fig. 1e–i), a birefringent, turbid, and viscous vesicle phase is observed. For the samples above 350 mM, POM observation shows Maltese crosses, indicating the existence of lamellar structures. A typical POM image is presented in Fig. 3. Within the vesicle region, samples with the concentration of [mim-C4-mim]Br2 above 375 mM become transparent and have birefringent textures that are different from the vesicles described above, indicating that the microstructure is different from the above vesicles. The samples in this region have a relatively higher viscosity than the samples of the above vesicle phase. However, the phase behavior of the SDBS/[mpy-C4-mpy]Br2/ H2O system is different from that of the SDBS/[mim-C4-mim] Br2/H2O system. As shown in Fig. 2, all of the samples are single phase. When the concentration of [mpy-C4-mpy]Br2 is 10 mM or
Rheological measurements Rheological measurements were performed on a Haake RS6000 rheometer with a coaxial cylinder sensor system (Z41 Ti). Dynamic oscillation-shear measurements were employed, in which the stress was varied while the frequency was kept at
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Fig. 1 Optical photographs of aqueous solutions with increasing amounts of [mim-C4-mim]Br2 and SDBS observed without (top) and with (bottom) crossed polarizers at 25 C.
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FF-TEM images of vesicle phase solutions for the SDBS/[mimC4-mim]Br2/H2O system: 350 mM (a) and 500 mM (b).
Fig. 4 Fig. 2 Optical photographs of aqueous solutions with increasing amounts of [mpy-C4-mpy]Br2 and SDBS observed without (top) and with (bottom) crossed polarizers at 25 C.
Cryo-TEM and FF-TEM images of vesicle phase solutions for the SDBS/[mpy-C4-mpy]Br2/H2O system: 50 mM (a, cryo-TEM) and 75 mM (b, FF-TEM). Fig. 5
Polarized micrograph for a typical sample of the 350 mM SDBS/ [mim-C4-mim]Br2/H2O system.
Fig. 3
lower (Fig. 2a), the solutions are clear with no birefringence, suggesting micelle solutions. When the concentration of [mpy-C4-mpy]Br2 is increased to about 50 mM, a turbid solution with birefringence appears. When the concentration is between 50 and 100 mM, a birefringent, turbid, bluish or ivory viscous phase is observed, indicating the appearance of a vesicle phase. Aer the vesicle region, samples with a concentration of [mpy-C4-mpy]Br2 above 100 mM become transparent and have birefringent textures different from that of the above samples. Microstructure observations and size distribution Cryo-TEM and FF-TEM are two powerful tools that can be employed to determine the self-assembled structure of surfactants in their hydrated states.37,38 For samples with low viscosity, cryo-TEM is a powerful tool to study the aqueous samples in their near-native hydrate state, while for samples with high viscoelasticity, FF-TEM is a more convenient tool for microscopical observation.39 The cryo-TEM and FF-TEM micrographs for selected samples are shown in Fig. 4 (SDBS/[mim-C4-mim] Br2/H2O system) and Fig. 5 (SDBS/[mpy-C4-mpy]Br2/H2O
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system). When the concentration of [mim-C4-mim]Br2 is 350 mM, polydisperse vesicles with diameters from about 50 nm to near 350 nm can be observed clearly (Fig. 4a). Slightly deformed vesicles can also be seen in the image, revealing the exibility of the vesicles, which is oen reported in hydrocarbon surfactant systems.40,41 When the concentration of [mim-C4-mim]Br2 increases to 500 mM, vesicles still mainly exist, which are densely packed (Fig. 4b). Similar to the SDBS/[mim-C4-mim]Br2/H2O system, vesicles can also be formed in the system of SDBS/[mpy-C4-mpy]Br2/ H2O. The cryo-TEM and FF-TEM observations for two selected vesicle phase samples are shown in Fig. 5. Cryo-TEM (Fig. 5a) shows that at the concentration of [mpy-C4-mpy]Br2 with 50 mM, vesicles with diameters about 200 nm are formed. When the concentration of [mpy-C4-mpy]Br2 increases to 75 mM, vesicles with diameters from about 100 nm to near 450 nm still mainly exist (Fig. 5b, FF-TEM image). Slight vesicle deformation can also be observed, indicating the exibility of vesicles.40,41 Dynamic light scattering (DLS) is utilized to further observe the aggregate size and morphology of the aqueous pseudogemini surfactant solution above the CMC (critical micelle concentration). All of the samples were chosen from the [mim-C4-mim]Br2-(SDBS)2 and [mpy-C4-mpy]Br2-(SDBS)2 systems described above. The DLS results are shown in Fig. 6, and the obtained hydrodynamic radii (R) of the aggregates from the DLS data are summarized in Table 1. Variations in aggregate size are observed when the concentrations of [mim-C4-mim]Br2
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concentration of [mpy-C4-mpy]Br2 increased. All of the sizes of the large aggregates are in agreement with the TEM results. However, as shown in Fig. S4,† the DLS results showed that only micelles with diameters lower than 20 nm formed in the SDS/[mim-C4-mim]Br2/H2O system and the SDS/[mpy-C4-mpy] Br2/H2O system. It can be concluded that the SDBS/[mim-C4mim]Br2/H2O system and the SDBS/[mpy-C4-mpy]Br2/H2O system experience a spontaneous phase transition from micelles to vesicles with increasing concentrations of the mixed solutions.
Rheological properties of the La-phase
Fig. 6 The size distributions of the [mim-C4-mim]Br2-(SDBS)2 (a) and [mpy-C4-mpy]Br2-(SDBS)2 (b) aggregates at different [mim-C4-mim] Br2 or [mpy-C4-mpy]Br2 concentrations. The hydrodynamic radius is plotted on a logarithmic scale.
Table 1 The obtained hydrodynamic radii (R) of [mim-C4-mim]Br2(SDBS)2 and [mpy-C4-mpy]Br2-(SDBS)2 aggregates versus the [mim-C4-mim]Br2 or [mpy-C4-mpy]Br2 concentrations
[mim-C4-mim]Br2-(SDBS)2 C (mM) R (nm)
1 7.0
5 8.6
10 15.8
350 232.6
425 220.2
500 182.7
10 13.6
25 64.8
50 113.5
75 214.7
[mpy-C4-mpy]Br2-(SDBS)2 C (mM) R (nm)
1 7.2
5 7.9
or [mpy-C4-mpy]Br2 increase. As can be seen from Fig. 6a, when the concentration of [mim-C4-mim]Br2 is lower than 10 mM, with increasing concentration of the mixture, the hydrodynamic radii (R) of the micelles increase slightly from 7.0 to 15.8 nm. When the [mim-C4-mim]Br2 concentration exceeds 350 mM, the hydrodynamic radius of the aggregates is about 200 nm. As for the SDBS/[mpy-C4-mpy]Br2/H2O system, the DLS results (Fig. 6b) show that micelles with a radius of about 10 nm formed when the concentration of [mpy-C4-mpy]Br2 was lower than 10 mM. Larger aggregates also formed when the
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Rheology is a powerful method not only to show the macroproperties of a surfactant system but also to conrm the microstructures of complex uids.42 Rheological measurements were carried out to characterize the macroscopic properties of the La-phase. The shear viscosity curves for the samples of the SDBS/[mim-C4-mim]Br2/H2O system in the La-phase as a function of shear rate are shown in Fig. 7. With increasing concentration of the pseudogemini surfactant, the viscosity of the solution becomes larger owing to the dense packing of the aggregates. It can be clearly seen from the gure that the viscosity decreases sharply with the increase of shear rate, showing emblematical non-Newtonian behavior of shear-thinning, which is typical for the La-phase.37,4344 Fig. 8 shows the dynamic rheological data for four selected samples at C[mim-C4-mim]Br2 ¼ 350, 400, 450 and 500 mM. As can be seen from the gure, the four samples have similar rheological properties. At C[mim-C4-mim]Br2 ¼ 350 mM, the storage modulus (G0 ) and loss modulus (G00 ) are nearly independent of the oscillatory frequency over the whole investigated frequency, exhibiting the obvious viscoelasticity and dominant elastic properties of the vesicle phase. When the concentrations of [mim-C4-mim]Br2 are 400, 450 and 500 mM, the storage modulus (G0 ) and loss modulus (G00 ) are slightly dependent on the frequency. Moreover, for all the selected samples, the storage modulus is almost 1 order of magnitude higher than the loss modulus. With increasing concentration of [mim-C4-mim] Br2, both the storage modulus (G0 ) and the loss modulus (G00 ) increase slightly. The complex viscosity rh*r decreases linearly with increasing frequency with a slope of 1. The rheological
Fig. 7 Viscosity as a function of shear rate for the vesicle phase
samples of the SDBS/[mim-C4-mim]Br2/H2O system: 350, 400, 450 and 500 mM.
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Fig. 8 Dynamic rheological data for the vesicle phase samples of SDBS/[mim-C4-mim]Br2/H2O: 350 (a), 400 (b), 450 (c) and 500 (d) mM.
results are consistent with the typical characteristics of densely packed vesicle solutions.45–48 Fig. 9 exhibits the rheological properties of the [mpy-C4-mpy] Br2 system at a [mpy-C4-mpy]Br2 concentration of 75 mM. Similar to the [mim-C4-mim]Br2 system, the solution demonstrates characteristic non-Newtonian behavior. Over the whole frequency, the storage modulus (G0 ) and the loss modulus (G00 ) 0 remain almost unchanged, and G is almost 1 order of magnitude higher than G00 . The complex viscosity rh*r decreases linearly over the whole frequency range with a slope of 1. In the SDBS/[mpy-C4-mpy]Br2/H2O system, vesicles are formed at a relatively lower concentration of the pseudogemini surfactant. Therefore, both the storage modulus and the loss modulus in the [mpy-C4-mpy]Br2 system are much lower.
Dynamic rheological data for the vesicle phase samples of the SDBS/[mpy-C4-mpy]Br2/H2O system: 75 mM.
Fig. 9
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Proposed mechanism of vesicle formation Surface tension measurements were carried out to explore the probable interactions between SDBS and [mim-C4-mim]Br2 or [mpy-C4-mpy]Br2. Surface tension curves of SDBS plotted against the surfactant concentration at different salt concentrations at 25 C are shown in Fig. S1.† The plots of the CMC and gCMC values for SDBS against the concentrations of added salts are shown in Fig. S2.† It is clear that with the increase of ionic strength, the CMC values of SDBS decrease sharply. A smaller value of gCMC means a greater ability to reduce the surface tension of a surfactant solution. As shown in Fig. S2(b),† the obtained gCMC values decrease sharply at rst and then change slightly with the increase of ionic strength considering the experimental errors. Therefore, the addition of [mim-C4-mim] Br2 and [mpy-C4-mpy]Br2 can distinctly reduce both the CMC and gCMC values. This result is mainly caused by the enhanced attractions between the head group of SDBS and [mim-C4mim]2+ or [mpy-C4-mpy]2+, including electrostatic, p–p stacking, and s–p interactions.5 The enhanced interactions can effectively reduce the electrostatic repulsions among the head groups of SDBS and then facilitate micelle formation.22 Very recently, Wang et al.32 have proved that in a 1,2-bis(2-benzylammoniumethoxy)ethane dichloride and sodium dodecyl sulfate (BEO/SDS) mixed aqueous solution, a (SDS)2-BEO gemini-type structure with enhanced aggregation ability can be fabricated through electrostatic binding between SDS and BEO. In the present SDBS/[mim-C4-mim]Br2/H2O and SDBS/[mpy-C4mpy]Br2/H2O systems, we infer that one [mim-C4-mim]Br2 or [mpy-C4-mpy]Br2 molecule may also bridge two SDBS molecules to form a pseudogemini surfactant molecule. As the
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concentration of the mixed solutions increases, the hydrophobic interactions among the SDBS alkyl tails become stronger, causing the alkyl tails to pack more tightly. As a result, a micelle-to-vesicle transition occurs when the concentration of the connecting molecule increases under the cooperative effect of electrostatic, p–p stacking, s–p, and hydrophobic interactions in the [mim-C4-mim]Br2/SDBS/H2O system and electrostatic, s–p, and hydrophobic interactions in the [mpy-C4-mpy] Br2/SDBS/H2O system. To further prove the weak interactions which induce the formation of vesicles, we also investigated the systems of SDS/[mim-C4-mim]Br2/H2O and SDS/[mpy-C4-mpy] Br2/H2O. The results show that all of the samples are clear, transparent, and homogeneous with no birefringence, indicating that only micelle solutions exist. Compared with SDBS, the surfactant SDS lacks a benzene ring on the molecule. 1 H NMR was carried out to explore the intermolecular interactions in the aqueous solutions of the pseudogemini surfactants. The 1H NMR spectra of the SDBS/[mim-C4-mim] Br2/H2O system at different concentrations in D2O at 25 C are shown in Fig. 10. It is clear that the protons around the headgroup move upeld slightly with the increase in concentration. Generally, the chemical shi of protons on surfactants moves downeld during aggregation in the absence of any other specic interactions.49,50 The upeld shis of the protons in the present system are probably caused by the p–p stacking. The addition of [mim-C4-mim]Br2 generates intermolecular p–p stacking between the phenyl and imidazolium rings, and the circular current makes the protons around the headgroup resonate at a higher eld due to the shielding effect.51 As the concentration of pseudogemini surfactant increases, the protons on the phenyl and imidazolium rings continue to shi upeld obviously, indicating the enhancement of p–p stacking interactions between the two aromatic rings. Meanwhile, the [mim-C4-mim]2+ may strongly combine with the SDBS headgroup, with the aromatic ring penetrating into the hydrophobic
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The possible mechanism of the phase transition for the pseudogemini surfactants system.
Scheme 2
cores of the pseudogemini surfactant aggregates, and then cause a phase transition from micelles to vesicles. In addition, some of the peaks broaden and even disappear, which is probably because the motion of the molecules is highly restricted in the vesicle phase.52,53 The packing parameter P is an important factor to interpret the self-assembled structure for a particular amphiphile. P is determined as P ¼ v/al by Israelachvili et al.54 where v is the effective hydrophobic chain volume, a is the effective headgroup area of the surfactant molecules, and l is the surfactant alkyl chain length. Surfactants with P below 1/3 tend to form spherical micelles, while P values between 1/3 and 1/2 prefer to form cylindrical aggregates and P values between 1/2 and 1 prefer to form bilayers. Surfactants with even higher values of P (P > 1) favor the formation of reverse structures. The calculated P value (Table S1†) is 0.56 for the SDBS/[mim-C4-mim]Br2/H2O system and 0.53 for the SDBS/[mpy-C4-mpy]Br2/H2O system, which are consistent with the formation of bilayer structures. With increasing concentration, the [mim-C4-mim]2+ cations would bind to the headgroup of SDBS strongly under the cooperative effect of electrostatic, p–p stacking and s–p interactions, and [mpy-C4-mpy]2+ cations would bind to the headgroup of SDBS under the cooperative effect of electrostatic and s–p interactions. Meanwhile, the hydrophobic interaction among the SDBS alkyl tails becomes stronger and makes the alkyl tails pack more tightly with increasing concentration. As a result, a phase transition from the micelle phase to the vesicle phase is induced at a certain concentration. According to the experimental results and analysis above, the possible mechanism of the phase transition from micelle phase to vesicle phase for the SDBS/[mim-C4-mim]Br2/H2O system and the SDBS/[mpy-C4-mpy]Br2/H2O system was drawn and is shown in Scheme 2. One can see that the strong interactions between SDBS and the cationic connecting molecule may provide the directional force for vesicle formation by pseudogemini surfactants.
Conclusions
Fig. 10 Partial proton assignments and 1H NMR spectra of the SDBS/ [mim-C4-mim]Br2/H2O system at different concentrations in D2O at 25 C.
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In summary, we have designed a kind of pseudogemini surfactants utilizing the anionic surfactant SDBS and the smallmolecule cationic spacers [mim-C4-mim]Br2 and [mpy-C4-mpy] Br2 at a molar ratio of 2 : 1. The presence of [mim-C4-mim]Br2 and [mpy-C4-mpy]Br2 induces vesicle formation in the
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pseudogemini surfactant system. We speculate that the electrostatic, p–p stacking and s–p interactions between SDBS and [mim-C4-mim]Br2, and the electrostatic and s–p interactions between SDBS and [mpy-C4-mpy]Br2 combined with the hydrophobic interaction among the SDBS alkyl tails contribute to the formation of vesicles. We believe that our research is helpful to achieve the controllability of organized assemblies by tuning specic weak interactions.
Acknowledgements The authors are grateful to the National Natural Science Foundation of China (no. 91127017), the National Basic Research Program (2013CB834505), Specialized Research Fund for the Doctoral Program of Higher Education of China (no. 20120131130003) and the Shandong Provincial Natural Science Foundation, China (ZR2012BZ001).
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